This is an introductory course for students with limited background in chemistry; basic concepts involved in chemical reactions, stoichiometry, the periodic table, periodic trends, nomenclature, and chemical problem solving will be emphasized with the goal of preparing students for further study in chemistry as needed for many science, health, and policy professions.

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From the lesson

Matter and Energy

<p>If you are interested in significant figures in more detail, here are some <a href="https://www.khanacademy.org/math/pre-algebra/decimals-pre-alg/sig-figs-pre-alg/v/significant-figures" target="_blank">good videos</a> to follow on Khan Academy.</p><p>This week we will continue our explorations of matter and energy. We will discuss the sub-atomic particles that govern chemical reactions, isotopes, anions, and cations. We will learn how to name compounds, calculate formula masses, convert between grams and moles, examine periodic trends, and more! An advanced problems set is posted now; that is a longer assignment and is optional unless you would like to be eligible for the Honor’s Track. You can still earn a regular verified certificate without completing the advanced problem sets, so please be sure to keep working on the normal weekly exercises.</p>

Taught By

Prof. Dorian A. Canelas

Transcript

[MUSIC] Welcome back for the second week of Introduction to Chemistry. Last week, matter was introduced, and you learned that matter occupies space and has mass. In this lecture, I'm going to focus in on atoms. And discuss the most chemically relevant subatomic particles, which are the smaller bits of matter that come together to make up individual atoms. Let's start by reviewing some of the concepts from last week that are the most important for this week. The first thing you've learned is that matter is composed of chemicals that can be composed and come classified as atoms, elements, molecules and compounds. Last week's lectures and exercises had quite a few examples of one facet of stoichiometry. This simple whole number ratios between atoms are different types of elements in a compound. Another thing that we discussed last week, that comes up frequently in chemistry and merits review, is Avogadro's Number. The quantity that is usually called the mole. Sometimes it's given the symbol NA, for Avogadro's number. Recall that this is simply a quantity, it's 6.022 times 10 to the 23rd. Recall that this is simply a quantity, it's 6.022 times 10 to the 23rd of anything. Now it just so happens that that quantity is very relevent in relating the masses of atoms to the masses of moles of atoms, and we'll talk about that later in this lecture. Here's a picture of Amedeo Avogadro, he's famous not only for giving us the mole, he's also famous for many important experiments that he did and laws that he wrote about such as a gas law that I'll discuss in future weeks lectures. Finally, during the explanation of scientific methods last week, we examined some elements of Dalton's atomic theory. One of the most important aspects of this theory. Is the concepts of atomic and molecular mass. This week's lecture will explore these concepts further, while we continue looking at the construction of the periodic tables of elements. So let's start with the concept of atomic mass. In the early 1800s, Dalton originally based atomic mass on hydrogen, the lightest element, which he gave a mass of 1. Since then, it's been changed to oxygen as the standard, where oxygen was given a mass of exactly 16. And then in the 60s, they decided to base it on carbon, to eliminate some errors that they were having when they were doing calculations. So now atomic mass is based upon carbon-12 is the standard. That's the arbitrary standard that people agree to use. Carbon is assigned a mass of exactly 12. All other atomic masses are relative to the mass of carbon. One-twelfth of a carbon atom, mass, is given a unit, the unified atomic mass unit, which has the symbol lower case u. You'll often also see that written as amu, for atomic mass unit. That's an older abbreviation that still gets used a lot. And in fact I still use it. This is also sometimes called the Dalton. Named after John Dalton for his important in theory in the concept of atomic mass. This gets used mostly in biology. For example someone might say, I have a 32 kilodalton protein. And what they mean is that their protein is 32,000 atomic mass units in mass, for a single molecule of that protein. So remember, one atomic mass unit has one-twelfth the mass of exactly one carbon 12 atom. On the periodic table, you'll see a box that has carbon in it. Carbon has six protons, so its atomic number is exactly 6. That's always the smaller number in the box on the periodic table. If you look at the periodic table, you'll see that all of the boxes, even on the most simple periodic table, have two numbers. The smaller number, which is always a whole number, is the atomic number. That tells us how many protons are in that atom. And that determines the identity of that element. So, for something to be carbon, we know that it has six protons in the nucleus. That's what the 6 comes from. If it had seven, then it wouldn't be carbon anymore. The other number that's on the periodic table is the average atomic mass. That number usually has decimal places after it, and it's not always a whole number. In fact, it's usually not, and some periodic tables go to more decimal places than others, depending on the level of precision that they're interested in presenting on that table. Avogadro's number is particularly useful in that it gives us some conversion factors that we can use to convert between atomic mass units and grams. Here's an example of a conversion factor. The relationship between atomic mass units and grams, is that one atomic mass unit has a mass of 1.6 times 10 to the minus 24 grams. Think about what that means. That means that 100 a hydrogen at, atom, for example, which weighs approximately one atomic mass unit, has a mass of approximately 10 to the minus 24 grams. So that's extremely, extremely light and very tiny, much smaller than things that we can think about weighing on the scales, for example, even something like a grain of sand weighs much more than that. Another conversion factor that's useful, and this is the magical part of the periodic table in Avogadro's number. That seems to throw people off. There's nothing actually magical or fancy about it. That is that one gram of a substance happens to weigh 6.022 times 10 to the 23rd atomic mass units. So these are both two conversion factors for the same thing. One of them has Avogadro's number as one of the numbers. But both of them are conversion factors. We could write them as a ratio. I could write 1 atomic mass unit over 1.6 times 10 to the minus 24 grams. Or I could write 1 gram over, over 6.022 times 10 to the 23rd atomic mass units. Those would just be different fractions I could write if I was doing unit conversions. So Avogadro's number remember, is also a quantity that we've called the mole. And that's abbreviated as M-O-L. Be really careful, not to abbreviate. Molecule as M-O-L or M-O-L-E. Sometimes students do that, but those both mean mole in chemistry and not the world molecule. Let's do some examples of how we can use the periodic table to do some simple calculations related to masses of atoms and masses of quantities of atoms. So go ahead and get out your handy dandy periodic table. I hope you always have one nearby, when you're working on this class. That's something that you're always allowed to use. I'd like you to without a calculator try to answer some of these questions. Let's do them together. So you may use a periodic table. But not a calculator for this. Go ahead and pause the video if you need to find a periodic table before we start. Okay, here we go. The first question is, what is the mass of one silver atom? Well, this is pretty easy to figure out, if you have a periodic table in front of you. You just find silver on your periodic table. And you'll see, that it has the larger of the two numbers in the box is 108. There's probably some decimal points after that, but let's just round it to 108 atomic mass units. That is the mass of a single silver atom. And it's a very tiny fraction of a gram. Remember that. Okay, now, let's do another question. So, that's the mass of a single atom. But, we can't actually take a pair of tweezers and grab a single atom, unless we have some extremely specialized equipment. So, if, in our life, we have, for example, a silver piece of jewelry, right? We might want to know what the mass is, of one mole of silver atoms. How much silver would we have to have, how big would the piece need to be in order for that piece of silver to contain 6.022 times 10 to the 23rd silver atoms? Well it turns out that you can just use the periodic table again. Because if one silver atom weighs 108 atomic mass units, then one mole of silver atoms weighs 108 grams. We actually don't use Avogadro's number very often in calculations because it's kind of built in to our agreed upon units. So one mole, of any of the elements on the periodic table, weighs that average mass that's given is the larger number in the box of underneath, it's usually underneath, the symbol of the element. All right, here's another question. How many silver atoms are in a 108 gram pile of silver? So, I'm asking the second question sort of in reverse. I've got a 108 gram pile of silver. How many atoms is that? Well the periodic table says that the average atomic mass of silver is 108. So, if I had 108 grams, that means I have one mole of silver atoms or 6.02 times 10 to the 23rd atoms. Do you see how this works? Okay. I'm going to give you a challenging one to do. Are you ready? This is for your own practice. What is the mass of a half of a mole of silver atoms? Go ahead and try that one now. Great. Let's move on to discussing the subatomic particles. Remember that atom, the word atom, came from the Greek atomos which meant indivisible. But, what exactly are atoms made of? They're all, all of the atoms of the different elements are slightly different, so there must be something different about their compositions. What do you think atoms are made of? Sometimes in class, I ask students to go ahead and try to sketch a drawing of what they think an atom looks like. And to describe what the atom is like. Is it hard? Is it squishy? So, in some sense, you might want to do, if you want to pause the video you can go ahead and do that. Just think about what you know about atoms, and try to draw a picture of an atom. As it turns out, atoms are mostly made of empty space. You can read about Rutherford's experiment in a variety of places, including on the internet. But, besides that empty space, which is the vast majority of the volume of an atom, there are a bunch of sub-atomic particles. And I'm going to zero in on three sub-atomic particles that are the most important for the chemistry of the elements. Here's a drawing of what that might look like. Now this drawing is not drawn to scale. That's the first thing that's really important to note. There's actually much more empty space than is shown there. In fact, the radius of the atom is really about a 100,000 times larger than the radius of the nucleus. And here I've made the radius of the atom. Not look that, that big compared to the size of the nucleus. Because I wanted you to be able to see the nucleus. So, I keep using this word nucleus. What is that? The atoms have a tiny dense center, where there are protons and neutrons. Particles called protons and neutrons. They're actually composite particles comprised of other elementary particles, but we aren't going to worry about those for now. We are just going to remember, that the atom in the center of it, has this tiny dense center sometimes people like to use a model for the atom. It's not actually a very good model, but it's one way to represent how we think about it, where they would say, well, our solar system has the sun in the center, right? And it's very dense, and then there are these planets orbiting it. That's a pretty simplified way of thinking about the atom, and it's not very accurate, but it does give you an idea. Of a model for having something in the center, where there are other things that are, sort of, orbiting around it. Perhaps, not in a, an elliptical or circular orbit like the planets, but in a more random way. The amount of space taken up by the nucleus is only about one ten-trillionth of the volume of the atom. So, the nucleus is extremely tiny and extremely dense, because that's where most of the mass of the atom is. We'll talk about that more in a minute. The electrons, which are shown here as little particles, move around in the space surrounding the nucleus. They're shown as particles, but we know that they actually have some wave particle duality, just like light has wave particle duality. The electrons weigh so little that they contribute practically no mass to the atom. Their contribution to the mass of the atom is nearly zero. They do weigh something, but it's extremely, extremely small compared to the protons and the neutrons. So most of the mass in the atom is in the nucleus. Most of the space of the atom is occupied by the electrons. The nucleus, it turns out, is positively charged. The protons have a positive charge, and the neutrons have no charge. So the nucleus, if we wanted to think about our model using Coulomb's law, Is like a dense spot where there is at least one positive charge in the case of hydrogen. But most elements have multiple positive charges in the nucleus. The electrons are attracted to the nucleus, because they have a negative charge. They're attracted by the force of Coulombic attraction, because they're oppositely charged from the nucleus. The amount of positive charge, or the number of protons, balances out the amount of negative charge, or the number of electrons in a neutral atom. Let's look more at the subatomic particles and talk about their masses and charges. So, this drawing, I just want everybody realize. This drawing is actually not particularly accurate. But it's a nice way to have a model to represent, what the difference of apom, tomic particles are in the nucleus. And it shows their relative location. Protons and nucle, protons and neutrons, excuse me, in the center and electrons occupying most of the volume outside of the center of the atom. The subatomic particles are all smaller than the atom itself of course. These can be further divided into classifications such as elementary or composite particles as I mentioned before. And if you're really interested in that, then I invite you to dive into the wonderful world of particle physics. For now though, I'm going to focus on the particles that are the most important for predicting the outcomes of chemical reactions and the chemistry of atoms and molecules. And the three particles that are the most important for that, are the electron, the proton and the neutron. These are the symbols that are often given to those particles, the electron is often represented with the symbol e minus. Protons sometimes will be represented with the symbol p plus. And the neutron sometimes will be represented with the symbol n0. This is the symbol that gets used the most. This symbol for the electrons. Because electrons do a lot of interesting things and chemical reactions. So, we find ourselves writing electrons down a lot. Now if we look at, what, makes up the mass of the atom. Because remember, an atom is the fundamental unit of matter in chemistry, and it has mass and occupies volume. So, if we look at. Okay, it has mass, but wh, where does the mass come from? Well, a tiny little bit of the mass comes from the electron, but you see, not very much. And I've written AMU there, and I know it really should be U, but AMU is kind of an old habit that dies hard. You'll see a lot of chemists write AMU for the mass unit, even though the new agreed upon international symbol is actually just a lower case u. The protons and neutrons, on the other hand have most of the mass. The neutron is very slightly heavier than the proton. But if we wanted to round it and not be too worried about very, very detailed precise calculations, we could just say both the proton and the neutron have a mass of about one. So, they're about one each. Okay, so the constant contributes to the mass of the proton and the neutron. If we look at the contributors to the charge on an atom, the charge comes from the number of electrons, each electron has a charge of minus one. Compared to the number of protons, each proton has a charge of plus 1. The neutrons have no charge, and they're actually there in the nucleus as a buffer to prevent all the positive charges of the nucleus from blowing it apart. So you can imagine, Coulomb's law says there should be a lot of repulsion between those protons, right? Because they're all positively charged. So, that the theory is that the neutrons act as a buffer and allow those protons to get together and be in that tiny dense center of the nucleus. So this atomic information is shown in different ways. On the periodic table, for example, you'll see a box that has the atomic number written above the element usually. Regardless of where it's written, it's always the smaller of the two numbers that's in the, on the periodic table box. And it's always the one that is a whole number. Another way sometimes you'll see atomic symbols written as this, this method here on the right. And in that case, the atomic number is usually written as a subscript before the element symbol. So, remember, the atomic number is the number of protons. That'd determines the identity of the element. So all iron atoms have 26 protons, if an atom doesn't have 26 protons then it is not iron, it is a different element. So, that's the first thing that protons do, is they determine the identity of the element. The other piece of information that's written on the periodic table, is this mass number. Here it's also shown in this symbol for the atom. Often written as a subscript before the element symbol there. But there is what's written on the Periodic Table is actually an average mass. So, there's a slight difference in what I have here for these two representations of iron. In the one that's in the box, and that's the one that I am trying to show you, is right off the Periodic Table. Right? because those are usually all in boxes. The mass, typically is some decimal place number. It's not a whole number. And that's because not all irons weigh exactly 56. For an Iron to weigh 56, let's calculate what we would need to have. Well we know we have 26 protons, we know that protons and neurons both weigh approximately 1. [SOUND] So, if the mass is 56, atomic mass units, and we have 26 protons, [SOUND] then an ion that has a mass of exactly 56 must have 30 neutrons. But not all ions have 30 neutrons, a lot of them do. But not all of them, some of them have fewer than 30 neutrons. That doesn't mean that they're not iron, remember the identity of the element comes from the number of protons. So, as long as there's 26 protons then that atom is an iron. But it could be short a neutron or maybe it has an extra neutron and that's okay, it's still iron. But, if we looked at all of the iron on earth and we averaged all of their masses, some of them have 30 protons, and some of them have fewer, and some of them might have more. If we averaged all of those masses from that population, we get this mass that's written on the periodic table. So, that when you're weighing a pile of iron out, you're going to have approximately the same atomic abu, relative atomic abundance of those different isotopes that come from having different neutrons, right? Different numbers and neutrons is going to give you something called different isotopes. We'll talk about that more in another lecture. Okay, but you could use this number, 55.9, to give you a good, accurate calculation for your sample of a lot of irons. because actually there is a little bit based on where you are on the planet, what the relative atomic abundance is, of the different isotopes. But for now we're just going to assume that what we can use, find on the periodic table, is accurate enough for the purposes of calculation. Now one thing that's not shown on this particular slide, is the number of electrons. How many electrons would a neutral iron atom have? In other words, if the entire Atom had no charge, overall. If the net charge was 0, how many electrons would it need to have? While it has 26 protons, right? So, it's got 26 positive charges. So, in order for it to be neutral, for it to have all of the positive and negative charges balanced out, a neutral iron atom, such as iron and a piece of steel, also has 26 electrons. Okay? Because then overall, that piece of iron would have no charge. If the iron had fewer than 26 electrons, and this happens a lot to iron, iron doesn't really have a problem losing some of its electrons, then that iron atom would have a positive charge. So, let's say that the iron atom had only 24 electrons. What would its charge be if an iron atom had only 24 electrons? That's great. Okay. Let's go back to recalling that the atomic number is the number of protons in the nucleus of an atom. And I want to emphasize again. I don't think I can say this enough times. That it determines the identity of the element. It's your turn now to look at the Periodic Table. What is the atomic number of the atom copper? So, first you have to remember what the symbol for copper is, do you remember what that is? Then you need to find it on the Periodic Table. And answer with the atomic number of copper, which remember is also the number of protons in a copper atom. I remember that when I first learned about chemistry, I had this question. I thought about Coulomb's law. Okay, and then we started talking about the structure of the atom and I thought to myself, okay, I know that like charges repel each other and have a pretty strong force of, force of repulsion at small distances. So, for example, how can beryllium have four protons stuck together in the nucleus if they're all positively charged? It seems like they would want to get away from each other and the lowest energy state would be for it to have those protons far apart from each other. In other words, since all the protons are positively charged, shouldn't they repel each other? Well, Marie Curie actually won a Nobel prize for this, and she had a theory that there was something called the nuclear force. And that nuclear force is stronger than the Coulombic repulsions. But, only at very small distances, so if things are very, very tightly packed, very, very close together, then the nuclear force can hold them together. And here's a picture of Marie Curie working in her lab. If you're really interested in this, I would encourage you to take the class in Nuclear Physics, hopefully there'll be one of those online. Sometime soon if there's not already. I should look to see. Maybe I'll take a class in nuclear physics. That would be fun. All right, so this has been a, a fun lecture. And let's end it by reviewing. Let's review the major subatomic particles that are important for the chemister. Remember there are more subatomic particles than the ones I'm listing here. Many more. My particle physicist friends would be upset if I said there were only three types of particles. But these are the ones that are important for the chemistry. And these are the only ones we're going to worry about at all in this class. The protons which determine the identity of the element. So, the number of protons determines whether this says carbon or is this silicon or is this phosphorous or is this calcium? That is determined by the number of protons. Remember all the protons have positive charges. [SOUND] The number of electrons then determines the net charge on the atom if it's neutral or the ion if it's not neutral. If there's more electrons than protons, then that species is negatively charged. [SOUND] So we have number of electrons greater than the number of protons, right? Then that's a negatively charged species. If the number of protons is greater than the number of electrons, just like I said earlier when we were talking about the iron had only had 24 electrons, then that's a positively charged ion, called a cation. And if the number of electrons is equal to the number of protons, which happens a lot, that is a neutral atom. Finally, the 3rd Atomic particle that we talked about were neutrons. The number of neutrons is what determines the mass of that particular atom. Since different atoms of the same type of element can have different masses. Okay? We call those isotopes. I'm going to talk more about ions and isotopes in a future video this week. So, check it out.

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